1. Field of the Invention
This invention relates to gaseous dielectric materials and to their use as insulators for electrical apparatus. More particularly, it relates to insulating media comprised of dielectric gas mixtures possessing unexpectedly high strength and to methods of calculating the proportions of such gas mixtures.
2. Description of the Prior Art
In many types of electrical apparatus electrical insulation is achieved by separating the conductors and utilizing atmospheric air as an insulator between them. But these measures may not be adequate for devices with large voltage gradients. Where electrical discharge does become a problem, insulation performance may be improved by using a gas whose dielectric strength is greater than that of air. Such performance may also be improved by increasing the density of the insulating gas, either by increasing its pressure or by lowering its temperature.
While increasing by pressure of a gas almost always increases its dielectric strength, use of this technique may also result in the need for a stronger vessel to contain the gas. Furthermore, pressurization can only be continued until the gas reaches its boiling point since any additional increase in pressure will cause the gas to liquify.
It is therefore desirable to utilize an insulating gas with a low boiling point since it can be more highly pressurized. Unfortunately, gases having the better dielectric strengths also tend to have high boiling points in comparison to gases of poorer strength. Indeed, at the operating temperature for a number of the electrical devices requiring insulation, many of the best dielectrics would be in a liquid state if employed individually.
Another consideration is cost. The better gaseous dielectrics tend to be the most expensive ones. This price differential becomes quite significant in such devices as transmission lines where the expenditure on gas represents a large percentage of the line's cost.
In order to provide a gaseous insulator of sufficient strength which will not liquify and satisfies budgetary constraints, a mixture of gases has often been employed. Thus, a high strength dielectric gas may be mixed with a poorer one of lower cost and/or lower boiling point to provide a mixture with a dielectric strength reported to be somewhere between the strength values for each of the two mixture components.
For example, in "Insulation Properties of Compressed Electronegative Gases" by P. R. Howard, Proceedings of IEE, 104A (1957), mixtures of the poor dielectric nitrogen, N.sub.2, and the higher strength sulfur hexafluoride, SF.sub.6, or dichlorodifluoromethane, CCl.sub.2 F.sub.2, were shown to have a dielectric strength somewhat above the mean of the strengths of the two components, each component being measured at the same temperature and pressure as that of the mixture. This was also the case in U.S. Pat. No. 2,221,670 where N.sub.2 and CCl.sub.2 F.sub.2 were mixed together. Of course, the insulating mixture may be comprised of more than two gases. In U.S. Pat. No. 3,281,521 a mixture comprised of N.sub.2, CCl.sub.2 F.sub.2 and SF.sub.6 exhibited a dielectric strength equal to that of SF.sub.6 at the same temperature and pressure. This was less than the strength of CCl.sub.2 F.sub.2, the best dielectric in the mixture.
This decrease in dielectric strength is an undesirable feature of gas mixtures employed as insulators. But because it has been believed that the dielectric strength of a mixture is always somewhere between the individual strengths of its components at the same temperature and pressure, decreased strength appeared to be an unavoidable consequence of adding a poor dielectric to a better one. This viewpoint can be seen, for example, in the third figure of British Pat. No. 791,205 where the dielectric strength of the claimed mixture is shown to be between that of its components, and it is stated at lines 62 through 77 of the patent that the dielectric strength of the mixture is higher than the expected mean of the two component gases.
With regard to the reported decrease in dielectric strength for gas mixtures, it should be noted that U.S. Pat. No. 3,506,774 does say in its abstract that mixtures of SF.sub.6 and perfluorovinyl sulfur pentafluoride possess a dielectric strength greater than either component alone. But the abstract fails to account for the two factors critical to such a statement. These factors are of course the temperature and pressure at which the mixture and its two individual components were measured. Since dielectric strength varies with changes in temperature and/or pressure, the better dielectric strength of the mixture is only significant if measured at the same temperature and pressure as that of both component gases. In order to understand what is meant by the abstract's statement, it is necessary to study both the specifications and claims of this patent. No such statement is found in either Section. While some of the dielectric strength values reported in Table III of the specification are slightly higher than those of the best dielectric component in that mixture whose performance is reported in Table I, no conclusion can be drawn from a comparison of these figures because the temperature and pressure of the component gas were not accurately measured in the experiments. As reported at column 2, lines 16 through 18 of the specification, the experiments were carried out at substantially one atmosphere and at about room temperature. Since the gas liquifies at approximately room temperature and the tests were carried out at that temperature, the gas density was probably difficult to maintain and measure during the experiments. The performance data thus cannot be compared with that of the mixture since there could easily be small differences in temperature or pressure between this component and the mixture and these differences would certainly account for the mixture's slightly higher discharge values.
As can be seen from this discussion of the prior art, an undesirable feature of previously reported gas mixtures has been their decreased dielectric strength in comparison to that of the best dielectric in the mixture. It is therefore one object of the present invention to provide insulating media comprised of a mixture of two or more dielectric gases wherein the dielectric strength of the mixture is not reduced by the presence of a poorer dielectric and is in fact superior to that of any component gas at the same temperature and pressure. Another object of the invention is to provide insulating media of high dielectric strength and low boiling point. A further object of the invention is to provide a method for determining which range of proportions of said component gases yield a dielectric strength superior to that of any component gas at the same temperature and pressure. Other objects, features and advantages of this invention will be obvious or will appear hereinafter.